(1) Field of the Invention
This invention relates generally to integrated circuits requiring accurate capacitance values. Specifically, the present invention introduces a method to use MOS devices as stable capacitors and comprises a method and circuits used for compensating capacitance variances of said MOS capacitors in integrated circuits in applications as oscillators, frequency references and capacitance references and a method to build capacitance references in integrated circuits.
(2) Description of the Prior Art
The capacitance of available capacitors in integrated circuits is varying more than 10% to 25%. Due to said variance of the capacitance applications requiring an accurate value of capacitance, as e.g. a suitable frequency reference cannot be implemented efficiently in integrated circuit. Therefore in integrated circuits said problems are solved in prior art by trimming e.g. an oscillator using fuses which is expensive and consuming tester time.
Typical applications where accurate values of capacitance are required are e.g. oscillators. Oscillators are circuits for converting dc power into a periodic wave-form or signal. Conventional RC oscillators advantageously furnish a low-cost timing source and allow for generation of variable frequencies by changing the resistance R, or capacitance C. Furthermore, conventional RC oscillators avoid advantageously the use of inductors, which are difficult to fabricate in integrated circuits.
Normally an RC relaxation oscillator needs a precision resistor R and also a precision capacitor C to achieve an accurate time constant T=Rxc3x97C. Since both devices vary by 10-25% in integrated circuit fabrication an expensive trimming scheme or external components must be used for such a frequency reference. FIG. 1 prior art shows as example the principle of such a relaxation oscillator. Said RC oscillator comprises the current source 1 with a current       Iref    =          Ur      R        ,
a periodical switch 2, a capacitor 3 having the capacitance C, a comparator 4 having as input the voltage Uc 21 at the capacitor C 3 and the threshold voltage Uth 4 and as output the voltage Us 5. Said threshold voltage is proportional to the reference voltage Ur.
Uth=kxc3x97Ur 
The principle is to charge a capacitor with a current Iref proportional to said reference voltage Ur. If the resulting voltage at the capacitor C 3 exceeds said threshold voltage Uth 4 a pulse or digital signal is created at the circuit output Us 5. Afterwards said capacitor C 3 is discharged/charged with the same current in order to initiate another switching event after a certain time which is defined by the switch 2. This is repeated continuously and periodically and therefore creates a constant frequency at the circuit output Us 5. FIG. 2 prior art shows the voltage Uc 21 at the capacitor C 3 (shown in FIG. 1 prior art) and FIG. 3 prior art shows the voltage at the circuit output Us 5.
The frequency f of said oscillator follows the formula   f  =            Iref              2        xc3x97        k        xc3x97        Ur        xc3x97        C              =          1              2        xc3x97        k        xc3x97        C        xc3x97        R            
It is obvious that the accuracy of the values of the resistor R and the capacity C have a direct impact to the frequency of said oscillator. Variations in the order of magnitude of 25% are not acceptable for most applications.
U.S. Pat. No. 6,020,792 (to Nolan et al.) describes a precision relaxation oscillator with temperature compensation. The precision relaxation oscillator is comprised of an oscillation generator comprised of a set-reset flip-flop and other components, a first current generator for producing a first output current and a second current generator for producing a second output current. The invention is implemented on a single, monolithic integrated circuit.
U.S. Pat. No. 5,801,411 (to Klughart) discloses an integrated capacitor structure having substantially reduced temperature and voltage coefficients including a combination of conventional N-depletion and P-depletion MOS gate capacitors connected in parallel and optimized for use at low bias voltages, where both the N-depletion and P-depletion capacitor structures have substantially zero temperature coefficients in their fully depleted region of operation.
U.S. Pat. No. 5,585,765 (to O""Shaughnessy) shows a low power RC oscillator including a low power bias circuit and an RC network. The RC network is used to form a time constant equal to the RC product. The RC oscillator includes a separate oscillator, such as a voltage-controlled oscillator (VCO), and uses the RC time constant to compare with the oscillator-generated period and to adjust the frequency of the overall RC oscillator circuit in accordance with the comparison. The RC oscillator is self-calibrating.
W. M. Sansen et al. (IEEE Journal of Solid State Circuits, Vol. 23, No. 3, June 1988) describe a temperature-compensated current reference for CMOS integrated circuits, based upon a MOSFET as current defining element. So as to minimize the mass production cost, it uses no external components nor trimming procedures. Comparison with classical current reference with a resistor as a current defining element shows a considerable improvement of the relative tolerances of the current.
A principal object of the present invention is to provide a method for compensating the variations of the capacitance of capacitors in integrated circuits.
A further object of the present invention is to achieve a frequency reference with MOS capacitors in integrated circuits.
Another further object is to achieve a capacitance reference using MOSFET capacitors in integrated circuits.
Another further object of the invention is to achieve a method to create a reference capacitor using an RC oscillator and a MOSFET capacitor in integrated circuits.
In accordance with the objects of this invention a method for tracking the MOS oxide thickness by the native threshold voltage Vth of a xe2x80x9cnativexe2x80x9d MOS transistor for the purpose of compensating MOS capacitance variations is achieved. Said method comprises, first, providing a MOS transistor having a native threshold voltage Vth. The first step is to sense the native threshold voltage Vth of said native MOS transistor, followed by building a suitable reference voltage level by adding by a circuit means another constant voltage Vx, then charging a MOS capacitor built out of a native MOS transistor up to the reference level Vref=Vx+Vth and finally measure a capacitance related value in a suitable configuration for oscillators which depends now no more on the oxide thickness.
In accordance with the objects of this invention a circuit for a frequency reference in an integrated circuit using a MOSFET as a stable capacitor is achieved. Said circuit, first, is comprising a constant current source, switching means charging and discharging said MOSFET capacitor being activated periodically by the output of comparing means charging and discharging said MOSFET capacitor. Furthermore said circuit is comprising comparing means having an input and an output wherein the input is the voltage of said capacitor and a voltage exceeding the threshold voltage of said MOSFET to influence the frequency and the amplitude of the output of said comparing means and the output is a periodical pulse. Furthermore the circuit comprises a MOSFET capacitor being charged/discharged periodically by said constant current source and its voltage level being the input to said comparing means and its threshold voltage is used to compensate the variations of capacitance, means to provide a voltage exceeding the threshold voltage of said MOSFET to influence the frequency and the amplitude of the output of said comparing means, and means to transform the periodical pulse into an alternating clock signal to drive said switching means and being the output of said frequency reference circuit.
In accordance to further objects of the invention a circuit to use a MOSFET capacitor as a reference capacitor in an integrated circuit is achieved. Said circuit, first, is comprising an RC-oscillator circuit having an input and an output wherein the input is a current source and a signal from a phase detector regulating the capacitance of said RC oscillator and the output is a periodical signal, a frequency reference circuit having an input and an output wherein the input is a current source and the output is a periodical signal and a phase detector circuit having an input and an output wherein the input are the periodical output signals from said RC oscillator and from said frequency reference and the output is a signal to regulate the capacitance of the RC oscillator and the capacitance of the capacitor to become a reference capacitor and said capacitor to become a reference capacitor.
In accordance to further objects of the invention a method for creating a capacitance reference in integrated circuits is achieved, said method is comprising, first, providing an RC oscillator circuit, a constant current source, a frequency reference, a phase detector and a capacitor to become a reference capacitor. The first step is generating with said RC oscillator circuit a periodic signal with frequency f1 followed by generating with said frequency reference circuit a signal with frequency f2, then comparing said frequency f1 and frequency f2 and regulating the capacitance of the capacitor of said RC oscillator until f1 equals f2, calculate the capacitance of the reference capacitor using an equation and use the calculated capacitance for a reference capacitor.